Eliminate name clash with system signal.h header file.

Tried to solve the signal.h name clash by using #include_next, but
seeing as this is not recommended[1] and breaks builds on some cross
compilers, the files were simply renamed: signal.[ch] --> sig.[ch]

[1] - http://gcc.gnu.org/onlinedocs/cpp/Wrapper-Headers.html

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

helpers.c

@@ -47,7 +47,7 @@
 #include "finit.h"
 #include "helpers.h"
 #include "private.h"
-#include "signal.h"
+#include "sig.h"
 
 #define NUM_ARGS    16
 #define NUM_SCRIPTS 128		/* ought to be enough for anyone */